Experimental investigation on two-phase flow instabilities of ultra supercritical pressure boiler with spirally water wall tubes

For a better design of 1000 MW ultra supercritical pressure boiler with spirally water wall tubes, an experimental study has been performed in the State Key Laboratory of Multiphase Flow in Power Engineering, Xi'an Jiaotong University to investigate the instability of the steam-water two-phase flow inside a smooth tube with an inclined angle of 20 degree from the horizon. The static hydrodynamic characteristic curves are obtained and the dynamic instabilities are also analyzed. The boundary conditions of the pressure drop oscillation and the density wave oscillation are proposed. The effects of pressure, mass flux, inlet and outlet throttle, inlet subcooling and the compressible volumes are analyzed primarily. The results demonstrate that the above parameters significantly influence the critical heat flux for the mentioned two types of oscillations and the density wave oscillations are the major type of oscillations in boilers and steam generators. The density wave oscillations occur at a higher heat flux than the pressure drop oscillations with the other operating parameters remaining constant. The inducing mechanisms of the above two types of oscillations are clarified. The variation ranges of critical heat flux and critical vapor quality are illustrated. Two empirical correlations predicting the onset of pressure drop oscillation and density wave oscillation are derived from the experimental data.